Manufacture of steel or alloy tubes



Patented Dec. 26, 1939 5 UNITED STATES PATENT OFFICE Fred 0. Baab andEdwin C. Wright, Beaver, Pa.,

assignors to National Tube Company, a corporation of New Jersey NoDrawing. Application December 3, 1937,

Serial No. 178,001

1Claim.

This invention relates to the manufacture of steel or alloy tubulararticles and, more particularly, to the manufacture of steel or alloytubular articles having improved machining qualities.

Tubular articles are now applied as substitutes for solid bar stock fornumerous uses, such as bearing races, chain links, roller cages andbushings, and such parts usually are machined on high speed automaticcutting machines. The parts thus manufactured from tubing must possessgood machinability in order to maintain a uniform product and highproduction rates, and it is desirable in many instances that themachined article have a good surface finish.

Investigation of many different kinds of steel manufactured in thetubular goods field and later machined on high speed automatic cuttingequipment, has demonstrated that the different steels require specialheat-treating and colddrawing operations in order to develop the bestcutting characteristics.

Manufacturing practice employed in making a tube from one grade of steelis often entirely unsuitable for making the tube from some other type ofsteel. It has been found that, by adding the improvement of slowcooling, medium carbon steels after heating the. pierced blank to atemperature above the upper critical point of the metal thereof prior tocold-drawing, the machining qualities of the finished tube are highlyimproved, both as regards tool life in high speed cutting operations andalso in the surface finish of the machined article It is accordingLv anobjectof the present invention to provide an improved method ofheattreating cold-drawn tubular articles during the process ofproduction.

It is a further object of our invention to pro-- vide a process forproducing tubular articles having' improved machining properties.

Cold-drawn seamless tubing is ordinarily produced by piercing a billetwhile hot, hot-rolling the pierced shell over a plug and thencolddrawing it to size. Prior to cold-drawing the shell is annealed and,if more than one cold-drawpass is necessary to obtain the desiredreduction, the shell is generally annealed between each cold-draw.

In accordance with the present invention which is particularlyapplicable to the production of tubing from medium carbon steels, i. e.,steels containing from .15 to .45 per cent. carbon and alloy steels ofthe same carbon content, the alloying agents of which may vary over awide range, a billet of such composition is pierced in accordance withconventional seamless practice. The pierced billet or shell is thenheated to a temperature above the critical range, slowly cooled,preferably in the furnace, as the rate of 5 cooling can be moreaccurately regulated, to a temperature substantially below the lowercritical point, and cold-drawn a predetermined amount to secure the bestresults. In order to secure the best machining qualities, which has beenfound to depend on the structural condition of the steel oralloy, adefinite structural state must be maintained. For medium carbon steeltubing containing from .15 to .45 per cent. carbon, regardless of thealloy additions, it has been found that the best structural state forfree cutting and smoothest finish is that which is produced by heatingthe steel to a temperature above the upper critical point, that is,between 1500 and 1750 degrees Fahrenheit, holding-it at this heat for atleastone hour, and then causing a slow furnace cooling at a rate notgreater than to 50 degrees per hour until the steel has reached atemperature in the furnace at least below 900 degrees Fahrenheit. Thehigher the temperature reached in heating and the slower the cooling,the better the structure obtained in medium carbon steels of the classdescribed. Slow cooling from above the upper critical range enables theexcess ferrite to precipitate from the 80 austenite in a uniformlydispersed condition, and also promotes the formation of the carbideconstituents in the form of lamellar pearlite. Steels with a carboncontent at the lower end of the above range, when so treated, usuallydevelop a banded structure of alternating bands of ferrite and pearlite.

After having been heat-treated in the manner described, the tube isready for the finishing colddraw pass. Cold-drawn tubes are, in conven-40 tional practice, cold-drawn to the desired size, but, in order toachieve the optimum results for machining on these medium carbon steels,the

reduction in area'should be between 20 and '70 per cent. The effect ofcold-drawing within these limits is to elongate, in the direction of thetube axis, the definite constituents which comprise the steel structure.The combination of the the slowly cooled structure and the cold-drawingpromotes the formation of two definite constituents, one hard and onesoft, which tends to give a continuous chip in cutting across theseelongated constituents and permits easier machining'and a smootherfinish. The cold-working reduces the elongation and reduction of." areaCarbon -perature below 900 degrees Fahrenheit.

Silicon of the steel, as measured in the conventional testing. Thiscombination of slow cooling and cold-working also tends to increase themachinability of the steel so treated.

As a specific example of producing tubing with a carbon content at thelower end of the above mentioned range our practice, as applied to thefollowing type of steel, known as S. A. E. 4620, is given:

Per cent Carbon .15- .25 Manganese .40- .70, Nickel 1.65-2.00 Molybdenum.20- .30 Silicon I- .15- .30 Phosphorus .04 mammum Sulphur .05 maximum Abillet of this analysis is pierced and rolled to produce a hot-forgedsection. The hot-forged tube is then heated to a temperature of 1675degrees Fahrenheit, held at that heat for at least one hour, and cooledin the furnace at a rate of 25 to degrees Fahrenheit per hour to a tem-The tube so treated is then pickled, lubricated and cold-drawn to obtaina per cent. reduction in area. The tube so produced has highly improvedcutting characteristics over tubing produced by any heretofore knownmethod.

When applied to steels having a carbon content at' the upper end of theaforementioned carbon range such as, for example, the type known as S.A. E. 3140 which has the following analysis:

Per cent .35- .45 Manganese .60- .90 Nickel 1.00-1.50 Chromium .45- .75.15- .30 Phosphorus .04 maximum Sulphur .05 maximum the tube is heatedto a temperature of 1550 degrees Fahrenheit and cooled in the furnace ata rate of 25 to 50 degrees per hour to a temperature below 900 degreesFahrenheit. The tube so treated is then pickled, lubricated andcolddrawn to obtain a 25 per cent reduction in area. It will'be notedthat the heat-treating temperature of this higher carbon content steelis somewhat lower than for the steel of lower carbon content abovementioned.

While we have described several embodiments ofour invention, it will beunderstood that we do not wish to be limited exactly thereto, sincevarious modifications may be made within the scope of our invention, asdefined by the appended claim.

We claim:

In the manufacture of cold-drawn steel tubing containing from .15 to .45per cent carbon, from .40-to .90 percent manganese, from 1.0 to 2.0 percent ni'ckel, from .20 to .75 per cent chromium, from .15 to .30 percent silicon, a maximum of .04 per cent phosphorus, a maximum of .05 percent sulphur and the remainder substantially iron to produce tubinghaving a high degree of machinability, the method comprising heating thetubular blank to a temperature of between 1500 and 1700 degreesFahrenheit, holding the blank at said temperature for approximately onehour, percipitating the excess ferrite from the austenite in a uniformlydispersed condition and promoting the formation of carbide constituentsin the form of coarse lamellar pearlite by slowly cooling said blank ata rate approximately of from 25 to 50 degrees per hour to a temperaturebelow 900 degrees Fahrenheit, and finally cold-drawing the blank anamount sufiicient to effect a reduction in area of between 20 and percent so as to form hard and soft layers therein which are freelymachinable. v

FRED C. RAAB.

EDWIN C. WRIGHT.

